This invention relates to a special Frequency Shift Keying (FSK) system that is known as the "Fast FSK", and in particular it provides an improved self-synchronizing receiver for demodulating "Fast FSK" radio signals.
The "Fast FSK" system uses a phase continuous FSK modulator where the two keyed frequencies differ by one half the bit rate. This modulation method is particularly suitable for the transmission of digital data over a noisy radio channel whose channel bandwidth is between 0.6 times and 0.9 times the bit rate of the data. For optimal demodulation of the data, coherent demodulation of an in-phase and of a quadrature channel is a necessity. In turn, coherent demodulation requires self-synchronization circuits for recovery of the clock reference signal and the r.f. carrier reference signal of correct frequency and phase. These reference signals are conventionally derived after a frequency doubling stage, as described in Canadian Pat. No. 1,066,371 entitled "Demodulator For Frequency-Shift Keying System" which was issued on Nov. 13, 1979 to R. de Buda, assignee to Canadian General Electric Company Limited.
A modulation system with similar properties as the Fast FSK system is the MSK system described in the publication "The Effect of Tandem Band and Amplitude Limiting on the E.sub.b /N.sub.o Performance of MSK" by H. R. Mathwich et al, IEEE Transactions Communications, Vol. COM-22, pp 1525-1540, October 1974. While the MSK system modulates a particular message differently, a random data stream creates for the MSK system the same power spectrum as for the Fast FSK system, and therefore, the same self-synchronization circuits can be used for either system. Hence, and without further reference, all the description which will be given for self-synchronization of the Fast FSK system should be understood to apply also to the self-synchronization of the MSK system.
Self-synchronization recovers the suppressed carrier from a signal in which all the transmitted power is used to transmit the messages so that the receiver must recover the carrier reference signal from the messages. Before carrier recovery has taken place, the messages must be considered as random data. Now it is known that random data, as transmitted by the Fast FSK system, represents a statistical process that has a continuous spectrum, and that such an unprocessed continuous spectrum is not suitable for carrier recovery, for which spectral lines are needed.
A bounded linear transformation of such a process having a continuous spectrum cannot give one with a line spectrum. Therefore, linear transformations alone (and this would include any combination of delays, multiplications with a fixed function, e.g. gating, and all filters) are not suitable for providing a spectrum from which self-synchronization can be achieved.
This shows that any method that can recover carrier and clock reference signals from a random data modulated Fast FSK, cannot be linear but must contain some non-linear circuit. A practical, simple and efficient method of providing such a non-linear circuit is to frequency double the signal in the self-synchronizing stage as described in the above Canadian Pat. No. 1,066,371, because, as it has been noted in the paper "Coherent Demodulation of FSK with Low Deviation Ratio", by R. de Buda, IEEE Transactions Communications, Vol. COM-20, pp. 429-435, the spectrum at the output of the frequency doubler contains two components: one is a continuous spectrum, the other a line spectrum with lines at frequencies 2f.sub.s and 2f.sub.m when the Fast FSK alternates randomly between the "mark" frequency f.sub.m and the "space" frequency f.sub.s, (f.sub.s &gt;f.sub.m). Two phase locked loops can now be used to extract the two lines from the frequency doubler output. The frequency doubling circuit in this case is then that non-linear circuit which, as discussed above, is an essential part of the self-synchronization circuits.
The oscillators in the phase-locked loops may be at 2f.sub.s, and 2f.sub.m but other combinations of oscillator frequencies are also possible. In particular, it may be of advantage to have one oscillator at f.sub.s +f.sub.m and the other at f.sub.s -f.sub.m, (which is a much lower frequency) and then to compare the output of the doubler with that of a mixer having the two oscillator signals as inputs and generating reference signals at:
(f.sub.s +f.sub.m).+-.(f.sub.s -f.sub.m)=2f.sub.s and 2f.sub.m. More complex frequency plans are possible. Also, it is not required to frequency double the incoming r.f. signal, which may be quite impractical, but the incoming signal could first be frequency shifted to a useful i.f. and there frequency doubled. If f is the frequency of the incoming signal and f.sub.o is the frequency of the local oscillator that is employed for frequency shifting to the i.f., then the above can be summarized by the identity:
2(f-f.sub.o)=2f-2f.sub.o, i.e. first frequency shifting by f.sub.o and then doubling is the same as first doubling and then frequency shifting by 2f.sub.o. These comments are included only for completeness of the description. They are state of-the-art and not novel.
In previously used circuitry, the frequency doubling is usually performed by a diode with square-law characteristic, or by a linear multiplier with the same signal applied to both inputs. In either case, second harmonics are generated as indicated by the identity: 2 cos.sup.2 .omega.t=1+cos 2.omega.t. A zonal filter rejects the dc and low frequency parts of the output and retains the second harmonic.
If however, the self-synchronization circuits are not designed with analog circuit elements, and the restriction is added that the circuit should be built mainly from logic gates, then it is found that many of the circuit elements can be readily replaced by gates alone. After all, any FSK signal is a constant amplitude signal, and the hard limiting which the gates provide retains the phase information.
However, the square of a hard limited signal is useless for this purpose, because it does not generate a second harmonic, since (-1).sup.2 =1, and a different self-synchronization method must be provided.